European Union regulators have blocked the planned merger of the New York and Frankfurt stock exchanges, a $10 billion deal that would have created the world's largest financial market operator.

The EU's Competition Commissioner, Joaquin Almunia, said Wednesday the proposed deal between the owners of the New York Stock Exchange and the Deutsche Boerse exchange in Frankfurt was rejected because it would have created an unfair, near monopoly in the trading of European derivatives. They are complex financial securities in which investors bet on the fluctuations in the value of such underlying assets as stocks, commodities and currencies.

Almunia said the new, combined exchange would have controlled 90 percent of the European derivative trading, which he said was not acceptable. He described trading in such securities as “at the heart of the financial system” and that it was crucial for the European economy that there be competition for the transactions.

The Wall Street exchange, often deemed the face of American capitalism, and Deutsche Boerse had announced the planned merger with great fanfare nearly a year ago, saying they needed to combine to save money and to offer their financial securities to a wider range of investors.

Deutsche Boerse's chief executive, Reto Francioni, called the EU's rejection of the merger “a black day for Europe and its global competitiveness on financial markets.”

The operator of the New York exchange, NYSE Euronext, and the Frankfurt exchange said they are discussing how to call off the proposed merger.

Financial analysts say the New York exchange is now likely to look for merger deals with smaller exchanges.

Some stock exchange mergers throughout the world have won regulatory approval. But several have been rejected as well, for the same reason as in the New York-Frankfurt case, that the combination would limit competition in the trading of securities.